Mechanical machining operations utilize a cutting tool to remove material from a workpiece to produce the desired finished geometry. Among the methods to accomplish mechanical machining, milling and drilling operations use a spindle to rotate the cutting tool that, when engaged with the workpiece, results in material removal. Turning, another method of mechanical machining, also uses a spindle to rotate the workpiece that, when engaged with the cutting tool, results in material removal.
The material removal process is dependent upon many parameters, including workpiece material, cutting tool material, lubrication, feed-rates and spindle speed. Spindle speed is important to produce the desired tangential velocity of the cutting tool (in the case of milling or drilling) or of the workpiece (in the case of turning). This tangential velocity, called the cutting speed, impacts the quality of the surface produced during the machining operation, and therefore must be tightly controlled to the desired setting for a given combination of workpiece and cutting tool materials. Furthermore, the spindle speed acts in conjunction with other machining parameters to determine characteristics such as chipload, that are also important to the performance of the machining operation. Several aspects of mechanical machining operations impart disturbance torques on the spindle and therefore affect the spindle speed, resulting in the requirement that the spindle-speed be actively controlled.
One form of speed control for a spindle is to use an electric motor and a motor drive to set the spindle speed. This method is robust but results in high costs, significant heat generation, and large mass. The cost, heat generation, and mass problems can be solved through the use of air-turbine driven spindles, however, the primary method to control the speed of these spindles, for high precision spindles, is through the modulation of the air pressure input to the spindle. This speed control method lacks accuracy and bandwidth due to the compressibility of air, the response time of control valves, and the speed at which the pressure signals reach the spindle (i.e., the speed of sound).
Air-turbine spindles offer several advantages including high rotational speed, low cost and high accuracy, but they lack high-performance (non-contact, high-accuracy, high-bandwidth) speed control functionality. Thus, it would be desirable to provide accurate, high-bandwidth speed control for air-turbine spindles.